The present invention relates to a station auxiliary power source apparatus that converts power supplied from an overhead cable to generate power to be supplied to a load in a station. The station auxiliary power source apparatus includes a first average calculation unit that calculates the average value of the overhead cable voltage for every predetermined time period; a second average calculation unit that calculates, two or more times, an average value of the overhead cable voltage that is larger than a first average value of the average value among the overhead cable voltages; a third average calculation unit that calculates an average value of the overhead cable voltage per day; and a comparator that selects any one of a second average value of the average value and a third average value of the average value and sets the selected value as the threshold value.